Implantable neurostimulation systems have proven therapeutic in a wide variety of diseases and disorders. Pacemakers and Implantable Cardiac Defibrillators (ICDs) have proven highly effective in the treatment of a number of cardiac conditions (e.g., arrhythmias). Spinal Cord Stimulation (SCS) systems have long been accepted as a therapeutic modality for the treatment of chronic pain syndromes, and the application of tissue stimulation has begun to expand to additional applications, such as angina pectoris and incontinence. Deep Brain Stimulation (DBS) has also been applied therapeutically for well over a decade for the treatment of refractory Parkinson's Disease, and DBS has also recently been applied in additional areas, such as essential tremor and epilepsy. Further, in recent investigations, Peripheral Nerve Stimulation (PNS) systems have demonstrated efficacy in the treatment of chronic pain syndromes and incontinence, and a number of additional applications are currently under investigation. Furthermore, Functional Electrical Stimulation (FES) systems such as the Freehand system by NeuroControl (Cleveland, Ohio) have been applied to restore some functionality to paralyzed extremities in spinal cord injury patients. Furthermore, in recent investigations Peripheral Nerve Stimulation (PNS) systems have demonstrated efficacy in the treatment of chronic pain syndromes and incontinence, and a number of additional applications are currently under investigation. Occipital Nerve Stimulation (ONS), in which leads are implanted in the tissue over the occipital nerves, has shown promise as a treatment for various headaches, including migraine headaches, cluster headaches, and cervicogenic headaches.
Each of these implantable neurostimulation systems typically includes one or more electrode carrying stimulation leads, which are implanted at the desired stimulation site, and a neurostimulator implanted remotely from the stimulation site, but coupled either directly to the stimulation lead(s) or indirectly to the stimulation lead(s) via a lead extension. Thus, electrical pulses can be delivered from the neurostimulator to the stimulation electrode(s) to stimulate or activate a volume of tissue in accordance with a set of stimulation parameters and provide the desired efficacious therapy to the patient. In particular, electrical stimulation energy conveyed to the electrodes creates an electrical field, which when strong enough, depolarizes (or “stimulates”) the neural fibers within the spinal cord beyond a threshold level, thereby inducing the firing of action potentials (APs) that propagate along the neural fibers to provide the desired efficacious therapy to the patient.
The combination of electrodes used to deliver electrical pulses to the targeted tissue constitutes an electrode combination, with the electrodes capable of being selectively programmed to act as anodes (positive), cathodes (negative), or left off (zero). In other words, an electrode combination represents the polarity being positive, negative, or zero. Other parameters that may be controlled or varied include electrical pulse parameters, which may define the pulse amplitude, pulse width, pulse rate, pulse shape, and burst rate. Each electrode combination, along with the electrical pulse parameters, can be referred to as a “stimulation parameter set.”
The neurostimulation system may further comprise a handheld patient programmer to remotely instruct the neurostimulator to generate electrical stimulation pulses in accordance with selected stimulation parameters. The handheld programmer in the form of a remote control (RC) may, itself, be programmed by a clinician, for example, by using a clinician's programmer (CP), which typically includes a general purpose computer, such as a laptop, with a programming software package installed thereon.
In the context of an SCS procedure, one or more stimulation leads are introduced through the patient's back into the epidural space, such that the electrodes carried by the leads are arranged in a desired pattern and spacing to create an electrode array. One type of commercially available stimulation leads is a percutaneous lead, which comprises a cylindrical body with ring electrodes, and can be introduced into contact with the affected spinal tissue through a Touhy-like needle, which passes through the skin, between the desired vertebrae, and into the epidural space. For unilateral pain, a percutaneous lead is placed on the corresponding lateral side of the spinal cord. For bilateral pain, a percutaneous lead is placed down the midline of the spinal cord, or two or more percutaneous leads are placed down the respective sides of the midline of the spinal cord, and if a third lead is used, down the midline of the spinal cord. After proper placement of the neurostimulation leads at the target area of the spinal cord, the leads are anchored in place at a spinal site to prevent movement of the neurostimulation leads. To facilitate the location of the neurostimulator away from the exit point of the neurostimulation leads, lead extensions are sometimes used.
The neurostimulation leads, or the lead extensions, are then connected to the IPG, which can then be operated to generate electrical pulses that are delivered, through the electrodes, to the targeted tissue. It is believed that the antidromic activation of the large diameter sensory neural fibers in the spinal cord (i.e., the APs propagate in a direction opposite to their normal direction, which in the case of the sensory neural fibers in the spinal cord, propagate in the caudal direction) provides the actual pain relief to the patient by reducing/blocking transmission of smaller diameter sensory neural fibers of the spinal cord via interneuronal interaction in the dorsal horn of the spinal cord, while the orthodromic activation of the large diameter sensory neural fibers in the spinal cord (i.e., the APs propagate in their normal direction, which in the case of the sensory neural fibers in the spinal cord, propagate in the rostral direction) generate APs that arrive at the thalamus and are relayed to the sensory cortex, thereby creating a side-effect in the form of a sensation known as paresthesia, which can be characterized as an tingling sensation. Thus, the sensation of paresthesia is concordant with the pain region of the patient, and typically must be present for pain mitigation.
Stimulation energy may be delivered to the electrodes during and after the lead placement process in order to verify that the electrodes are stimulating the target neural elements and to formulate the most effective stimulation regimen (i.e., the best stimulation parameter set or sets). The stimulation regimen will typically be one that provides stimulation energy to all of the target tissue that must be stimulated in order to provide the therapeutic benefit, yet minimizes the volume of non-target tissue that is stimulated.
Intra-operatively (i.e., during the surgical procedure), the neurostimulator may be operated to test the effect of stimulation and adjust the parameters of the stimulation for optimal pain relief. The patient may provide verbal feedback regarding the presence of paresthesia over the pain area, and based on this feedback, the lead positions may be adjusted and re-anchored if necessary. A computer program, such as Bionic Navigator®, available from Boston Scientific Neuromodulation Corporation, can be incorporated in a clinician's programmer (CP) (briefly discussed above) to facilitate selection of the stimulation parameters. Any incisions are then closed to fully implant the system. Post-operatively (i.e., after the surgical procedure has been completed), a clinician can adjust the stimulation parameters using the computerized programming system to re-optimize the therapy.
While the electrical stimulation of the spinal cord has generally been successful in providing pain therapy to a patient, the side-effect of paresthesia that typically accompanies the pain therapy limits the amplitude of the electrical stimulation that can be applied to the targeted spinal cord region. That is, although an increase in the amplitude of the electrical stimulation may optimize mitigation of the pain in a targeted region of the patient, such increase in the amplitude may undesirably result in paresthesia that is too intense for the patient to tolerate. As a result, the pain experienced by the patient may not be eliminated or otherwise reduced to a sufficient level that might have otherwise occurred in the absence of the paresthesia.
Furthermore, because the target neural tissue (i.e., the tissue associated with the therapeutic effects) and non-target neural tissue (i.e., the tissue associated with undesirable side-effects) are often juxtaposed, therapeutically stimulating neural tissue while preventing side-effects as a result of stimulating non-target neural tissue may be difficult to achieve. For example, electrical stimulation of the spinal cord may evoke action potentials that ultimately lead to subsequent action potentials propagated in motor neural fibers via motor reflex spinal cord loops, which may, in turn, lead to undesirable outcomes (e.g., discomfort or involuntary movements) for the patient. Thus, inadvertent stimulation of non-target neural tissue may also limit the amplitude of the electrical stimulation that can be applied to the targeted spinal cord region.
There, thus, remains a need to stimulate targeted spinal cord tissue at an amplitude sufficient to optimize therapy for the patient without creating side-effects, such as uncomfortable paresthesia and involuntary motor movements.